In order to provide an economically useful nitric acid extraction process for alumina, the nitric acid must be recovered from the total process in sufficient quantity as to provide a high percentage of acid recirculation. It is known in the art that substantial nitric acid may be recovered by direct condensation of HNO.sub.3 in the decomposition step of such a process. Such recovery recycles about 67% of the nitric acid used in the process. However, substantial amounts of the acid exits other stages of the decomposition in the form of NO.sub.X gases.
The recovery of nitric acid solutions from nitrous gases produced by the catalytic combustion of ammonia in air is a well-known art that is practiced commercially around the world. The basic process comprises contacting the ammonia oxidation gases at a pressure of 3 to 6 atmospheres absolute, or even higher, in a bubble cap-tray absorption column containing on the order of a hundred trays in countercurrent relationship with a supply of water introduced at the top of the column. Variations of the technology are concerned substantially with design of the bubble cap trays, oxidation of the ammonia under pressure, or oxidation at about atmospheric pressure to reduce catalyst consumption followed by the compression of the cool gases, the recovery and re-use of the heat produced in the ammonia oxidation reaction, and recently methods of reducing the approximately 1,000 ppm NO.sub.X concentration in the tail gas before releasing this gas to the atmosphere.
The chemistry of the conversion of NO.sub.X gases to nitric acid in solution is generally considered as consisting of 2 overall reactions which serve to define the mass balance between the liquid and vapor streams. Reaction 1: 3NO.sub.2 (g)+H.sub.2 O(1)=2HNO.sub.3 (aq)+NO(g) which occurs principally in the liquid phase and reaction 2: 2NO(g)+O.sub.2 (g)=2NO.sub.2 (g) which occurs substantially in the gas phase. The rate of reaction 1 is thought to depend primarily upon the rate of absorption of NO.sub.2 into the liquid stream which depends upon the partial pressure of NO.sub.2 in the gas stream and is thus slowed down by the presence of large quantities of inert gases such as N.sub.2, and concentrations of NO in the gas phase which tend to drive reaction 1 in the reverse direction. Once the NO.sub.2 has been absorbed the reactions in the liquid phase appear to proceed at satisfactory rates. The rate of reaction 2 is proportional to the product of the square of the partial pressure of NO and the partial pressure of O.sub.2 and can be quite slow in the presence of large amounts of inert gases such as N.sub.2. In the ammonia oxidation process for making nitric acid the feed gases from the oxidizer comprise on the order of 70 volume percent N.sub.2 and the proportion of N.sub.2 increases as the NO.sub.X gases are absorbed from the gaseous stream. Additional N.sub.2 is added with air to provide some oxygen in the tail gas to drive reaction 2 toward completion. Thus, although nitric acid has been recovered from ammonia oxidation gases at about atmospheric pressure using 2 or 3 absorption towers in series it has been found more economical to compress the gases to 3 to 6 atmospheres absolute so as to increase the partial pressure of the reacting gases sufficiently to permit carrying out the reconstitution in a single tall column.
The thermal decomposition of metal nitrates to oxides produces NO.sub.X gases together with at least the stoichiometric amount of O.sub.2 required to convert the NOX gases to HNO.sub.3. Decomposition of multivalent metal nitrates, for instance Pb(NO.sub.3).sub.2, Ca(NO.sub.3).sub.2, UO.sub.2 (NO.sub.3).sub.2 nH.sub.2 O, Al(NO.sub.3).sub.3 nH.sub.2 O, Al(OH).sub.2 NO.sub.3 nH.sub.2 O--produce the stoichiometric requirement of O.sub.2, i.e., 1 mol of O.sub.2 per 4 mols of (NO+NO.sub.2) plus 1/2 mol O.sub.2 per mol of any NO formed. Monovalent metal nitrates, such as NaNO.sub.3, may yield more than enough O.sub.2 because portions of the contained nitrogen values may be converted to N.sub.2 gas during decomposition instead of NO.sub.X. When the thermal decomposition is carried out in properly constructed, indirectly-heated decomposers that restrict the ingress of air, the NO.sub.X gases contain little or none of the inert N.sub.2 and are therefore highly concentrated in reactive components. A typical composition of such an NO.sub.X gas resulting from the thermal decomposition of aluminum nitrate by a decomposition process is disclosed in co-pending U.S. Pat. application Ser. No. 61,297, shown in Table 1, Column 2, is 25 volume percent of (NO+NO.sub.2), 12.5 volume percent O.sub.2 and 62.5 volume % H.sub.2 O. Since, in an absorption column, water vapor is absorbed into the liquid stream much more rapidly than NO.sub.X, the concentration of the reacting gases increases during passage through the absorption column so that the same or even higher rates of Reactions 1 and 2 may be achieved as can be achieved with ammonia oxidation gases at elevated pressures.
Since both Reactions 1 and 2 are highly exothermic and the easily reversible Reaction 1 can begin converting HNO.sub.3 from the acid solution to NO.sub.2 in the gas at temperatures as low as 150.degree. to 180.degree. F., depending upon the concentration of HNO.sub.3 in the liquid and of NO in the gas phase, the removal of heat from the absorption column is of major importance. It is known in the art to remove this heat either by placing water-cooled cooling coils in the liquid layer maintained on the upper side of the bubble cap trays or to withdraw a portion of the liquid from each of a number of trays in the column, pass the liquor through individual heat exchangers, and return it to the column after cooling. Plants handling ammonia oxidation gas typically provide sufficient cooling to the column by one or the other means so that the strong acid exiting the column is cooler than about 120.degree. F., or even lower depending upon the strength of the nitric acid that is being manufactured.
In contrast to the recovery of nitric acid from ammonia oxidation gases there has been very little need around the world to recover nitric acid from concentrated NO.sub.X streams such as that described above for the decomposition of aluminum nitrate materials.